TWI911358B - Electrolyser for electrochlorination processes and a self-cleaning electrochlorination system - Google Patents

Abstract

The present invention concerns a chlorination electrolyser comprising, a housing provided with an inlet and an outlet suitable for the circulation of brine; at least one pair of bipolar electrodes facing each other and positioned within said housing. The electrolyser is characterised in that each bipolar electrode of said at least one pair comprises: a valve metal substrate; an active coating comprising at least one layer of a catalytic composition comprising ruthenium and titanium disposed over said substrate; a top coating comprising at least one layer of a composition comprising oxides of tantalum, niobium, tin, or combinations thereof disposed over said active coating. The invention also concerns a self-cleaning electrochlorination system comprising such an electrolyser, a method for its production, its use in normal and low salinity pools for hypochlorite mediated water disinfection and a method for hypochlorite-mediated water disinfection.

Description

Electrolyzers and self-cleaning electrochlorination systems used in electrochlorination processes.

This invention relates to the operation of a chlorination electrolyzer under polarity reversal conditions, a method for manufacturing the chlorination electrolyzer, and a self-cleaning electrochlorination system.

Electrochlorination involves producing hypochlorite from brine via an electrolytic reaction. The resulting sodium hypochlorite can be used in various applications related to water disinfection and oxidation, such as drinking water treatment and microbial control in swimming pools or cooling towers.

Sodium hypochlorite is effective against bacteria, viruses, and fungi, and has the advantage that microorganisms cannot develop resistance to its effects.

Unlike chlorine gas or chlorine tablets that can be added to water to achieve similar results, in electrochlorination, the active chemicals are produced on-site, thereby avoiding transportation, environmental, and/or storage problems. The method is carried out by applying a suitable current to an electrolytic cell comprising at least two electrodes and an electrolyte containing brine, a mixture of salt and water, with concentrations varying depending on the application. The electrochemical reaction results in the production of sodium hypochlorite and hydrogen gas.

Titanium electrodes with active coating compositions (containing a mixture of valve metals and noble metals, particularly rare transition metals from the platinum group) have been successfully used as anodes in these types of batteries in the past. However, over time, scale forms on the active surfaces of the electrodes, negatively impacting the hypochlorite production efficiency of the electrolyzer.

To prevent/reduce scale formation, the electrodes can be periodically reversed to promote self-cleaning. Reversing the polarity also reduces ion bridging between the electrodes and prevents uneven electrode wear.

Under polarity reversal conditions, where each electrode alternately functions as both cathode and anode, some elements occasionally used in the active coating composition become unstable and dissolve in the electrolyte after a few reversal cycles, thus resulting in insufficient electrode lifetime.

Generally, reversing polarity for the active coating of electrodes is a harmful operation, as it rapidly leads to deactivation due to peeling.

To mitigate these issues, bipolar electrodes used under polarity reversal conditions require a higher coating load than when each electrode operates solely as an anode or cathode. Typically, electrode durability depends on both the polarity reversal frequency and the coating load.

Increasing the coating load negatively impacts electrode costs, both in terms of material quantity and the length of the production process. Furthermore, since many active coating compositions rely on rare transition metals with limited availability, the increased load exacerbates any related procurement challenges.

The desired feature is a self-cleaning electrode for use in electrochlorination systems, exhibiting improved lifespan and efficiency across a wide range of potential applications and operating conditions, while potentially maintaining reduced production costs. Furthermore, it is desirable to use this type of electrochlorination system in both normal and low-salinity ponds, i.e., ponds with salinity equal to or lower than 6 g/L (typically, sodium chloride (NaCl) concentrations are 0.5 to 2.5 g/L in low-salinity applications and 2.5 to 4 g/L in normal-salinity applications).

World Patent Application No. WO2019/215944A1 discloses an electrolytic cell for ozone production, equipped with electrodes having a thick dielectric surface layer to increase oxygen overvoltage at localized noble metal sites in the intermediate layer for oxygen production. These electrodes are unsuitable for chlorine production and also unsuitable for operation under polarity reversal conditions.

This invention relates to a chlorination electrolyzer, comprising a housing having an inlet and an outlet suitable for brine circulation, and at least one pair of bipolar electrodes facing each other and positioned within the housing. Each bipolar electrode comprises: (i) a valve metal substrate; (ii) an active coating including at least one catalytic composition disposed on the substrate, the catalytic composition including ruthenium and titanium; and (iii) a top coating including at least one composition comprising oxides of tantalum, niobium, tin, or combinations thereof and positioned on the active coating.

On the other hand, the present invention relates to a self-cleaning electrochlorination system, comprising: (i) the aforementioned electrochlorination cell; (ii) an electrolyte comprising an aqueous solution of sodium chloride (NaCl) salt circulating within the electrolytic cell; and (iii) an electronic system for periodically reversing the polarity of a bipolar electrode pair electrically connected thereto, and positioned outside the casing of the electrolytic cell.

On the other hand, the present invention relates to a method for manufacturing a chlorination electrolyzer according to the present invention.

On the other hand, this invention relates to the use of the aforementioned chlorination electrolyzer in normal and low salinity tanks for hypochlorite-mediated water disinfection.

On the other hand, the present invention relates to a method for hypochlorite-mediated water disinfection using the aforementioned chlorination electrolyzer under polarity reversal conditions.

In one aspect, the invention relates to a chlorination electrolyzer, comprising a housing having an inlet and an outlet suitable for brine circulation; and at least one pair of bipolar electrodes, facing each other and positioned within the housing, wherein each of the pair of bipolar electrodes comprises: (i) a valve metal substrate; (ii) an active coating including at least one catalytic composition disposed on the substrate, the catalytic composition comprising ruthenium and titanium; and (iii) a top coating including at least one composition disposed on the active coating, the composition comprising oxides of tantalum, niobium, tin, or combinations thereof.

The at least one catalytic layer comprising ruthenium and titanium is substantially homogeneous in terms of its electrical properties. The at least one catalytic layer is also homogeneous in terms of its morphological properties and constitutes substantially a solid solution comprising ruthenium and titanium, preferably a homogeneous solid solution, wherein the metals are primarily oxides, namely ruthenium oxide and titanium oxide.

The chlorination electrolyzer of this invention can be used for hypochlorite-mediated water disinfection in various applications, such as water tanks and wastewater disinfection (e.g., municipal water treatment, blackwater and greywater treatment, seawater chlorination, etc.).

Chlorination electrolyzers can advantageously operate under polarity reversal conditions, thus ensuring self-cleaning of the electrodes and preventing scale formation.

Each electrode of the pair can be coated on one or both sides. By convention, the two opposing electrodes should be arranged so that the coated sides face each other.

A chlorination electrolyzer may include a plurality of bipolar electrode pairs, forming coated electrodes stacked in a generally parallel configuration.

The housing should be designed to allow electrical connection of the bipolar electrodes to an external generator. The generator can advantageously be equipped with a system that reverses the electrode polarity at a preset frequency, typically within the range of 30 minutes to 10 hours, depending on the application and operating conditions such as water contaminants and water hardness, as is well known in the art.

Valve metal substrates can be any geometry commonly used in this field, such as (but not limited to) flat plates, perforated plates, screens, and louvers. Titanium is preferred as the substrate due to its durability, cost, and ease of surface preparation.

Before applying an reactive coating, it is best to clean, sandblast, and etch the substrate to ensure proper adhesion.

The reactive coating can be directly applied to the valve metal substrate using roller coating, brush coating, and spray coating techniques. Alternatively, this invention allows for the insertion of an intermediate coating between the substrate and the reactive coating, for example, to improve the adhesion of the reactive coating. In this case, the intermediate coating should still be considered as being applied to the substrate, albeit indirectly.

In one embodiment, the catalytic composition of the chlorination electrolyzer according to the invention, expressed as a weight percentage of elements, comprises 25% to 45% ruthenium and 55% to 75% titanium.

In another embodiment, the catalyst composition may, where appropriate, include 2% to 5% of dopant elements selected from the group consisting of ctanium, strontium, ferrodium, bismuth, zirconium, aluminum, copper, rhodium, iridium, platinum, palladium, and combinations thereof. These dopants can advantageously contribute to improving the lifespan of the chlorination electrolyzer and the efficiency of free available chlorine leaching.

Applying an insulating top coating of tantalum, niobium, or tin oxide (combined or alone) over the active coating according to any of the above embodiments allows the ruthenium (Ru) load to be reduced to 38% for the given lifetime target of the electrode without affecting efficiency.

Due to the scarcity of ruthenium and the associated procurement and cost issues, especially compared to the metal oxides used in the topcoat composition of this invention, the reduced ruthenium (Ru) loading offers a significant advantage.

The inventors have found that tin oxide topcoats are particularly effective in embodiments of the invention because tin (Sn) appears to form an oxide that allows chloride ions (Cl- ⁾ to diffuse more effectively into the active layer compared to tantalum (Ta) or niobium (Nb). Tin (Sn) topcoats also create surfaces with fewer cracks because they have a lower tendency to form delaminations, which, for example, cause the typical cracks that can be observed on tantalum oxide surfaces. The fewer cracks prevent electrolytes from dissolving unstable portions of the active layer.

In another embodiment, the top coating is preferably thin enough (between 0.5 and 7 micrometers) because it helps maintain the free available chlorine (FAC) efficiency of the active layer.

In any of the above embodiments, the reactive coating can have a ruthenium load of 1 to 30 g/m², which is effective for applications with salinity higher than 6 g/L (but preferably lower than 30 g/L), such as seawater chlorinators, and for applications with salinity lower than 6 g/L, such as those found in pools, ranging from 0.5 to 4 g/L.

In pool applications, the optimal total load capacity for the topcoat is 2 to 6 grams per square meter.

Without limiting the invention to any particular theory, the top coating of the invention forms a network rather than a barrier: it reduces mechanical wear on the surface of the active coating caused by bubble friction and retains partially dissolved material during polarity reversal, thereby preventing coating peeling and the dissolution of ruthenium and other optional impurities in the electrolyte. Simultaneously, the porosity and thinness of the top coating allow the electrolyte to reach the catalytic centers of the active coating.

On the other hand, the present invention relates to a self-cleaning electrochlorination system comprising: (i) the aforementioned electrochlorination cell; (ii) an electrolyte comprising an aqueous solution of sodium chloride (NaCl) salt at a concentration of 1 to 30 g/L circulating within the electrolytic cell; and (iii) an electronic system for periodically reversing the polarity of the bipolar electrodes of the electrolytic cell, the electronic system preferably being positioned outside the casing of the electrolytic cell and electrically connected to the bipolar electrodes.

The electronic system for periodically reversing the polarity of bipolar electrodes is equipped with an internal clock, allowing the polarity of the bipolar electrodes to be reversed at preset time intervals (within the range of 30 minutes to 10 hours).

In pool applications, the inventors have observed that the self-cleaning electrochlorination system of the present invention performs particularly well when the polarity of the bipolar electrode pairs in the electronic system is reversed at preset intervals of 1 to 4 hours.

Stacks comprising 5 to 15 parallel bipolar electrode pairs have been found to be advantageous for the implementation of this invention.

The electronic system according to the invention can advantageously operate at current densities of approximately 200 to 600 amperes per square meter (preferably 200 to 400 amperes per square meter).

On the other hand, the present invention relates to a method for manufacturing the aforementioned chlorination electrolyzer, comprising the steps of manufacturing each of the at least one pair of bipolar electrodes according to the following sequence of steps:

a) An active coating solution comprising precursors of both ruthenium and titanium is applied to a valve metal substrate to obtain a coated substrate;

b) Bake the coated substrate at a temperature of 450 to 550°C for 2 to 10 minutes;

c) Repeat steps a) and b) until the desired ruthenium load is achieved;

d) Applying a topcoat solution comprising a precursor material containing tantalum, niobium, tin, or a combination thereof to a coated substrate;

e) Bake the coated substrate at a temperature of 450 to 550°C for 2 to 10 minutes;

f) Repeat steps d) and e) until the desired load of tantalum, niobium, tin, or a combination thereof is achieved; and

g) Perform final heat treatment within a temperature range of 450 to 550°C.

The precursors of ruthenium and titanium, and precursors of tantalum, niobium, or tin, are compounds selected from the group consisting of methanol salts, ethoxides, propoxides, butoxides, chlorides, nitrates, iodides, bromides, sulfates, or acetates of such metals, and mixtures thereof.

Depending on the situation, the coated substrate may be air-dried at a temperature of 20 to 80°C for 2 to 10 minutes after step a) and/or after step d).

Typically, the chlorination electrolyzer according to the present invention (particularly with regard to the bipolar electrode structure) can be successfully applied in all hypochlorite production applications undergoing polarity reversal to reduce the noble metal load on the active coating or, if the same load is applied, to exhibit extended lifespan without affecting the free available chlorine (FAC) efficiency.

The inventors have discovered that chlorination electrolyzers operate particularly well in pool applications (operating at salinities of 0.5 to 4 g/L).

In another aspect, the objective of this invention is to utilize the chlorination electrolyzer of this invention for hypochlorite-mediated water disinfection in normal and low salinity water tanks, specifically for use in water tanks operating at salinity equal to or lower than 6 g/L (in low salinity applications, sodium chloride (NaCl) is typically 0.5 to 2.5 g/L, and in normal salinity applications, it is typically 2.5 to 4 g/L).

The following embodiments illustrate specific ways in which the invention is implemented, and their practicality has been largely verified within the scope of the claimed protection.

This invention also relates to a method for hypochlorite-mediated water disinfection, comprising the following steps:

a) The electrolyte comprising an aqueous solution of sodium chloride (NaCl) salt at a concentration of 1 to 30 g/L is circulated in at least one chlorination electrolyzer as defined above, the chlorination electrolyzer comprising one or more pairs of bipolar electrodes;

b) Applying a current to the bipolar electrode pair to produce hypochlorite in the sodium chloride (NaCl) salt aqueous solution; and

c) The polarity of at least one pair of bipolar electrodes is periodically reversed during the application of the current.

According to embodiments of the present invention, the polarity of the at least one pair of bipolar electrodes is reversed at time intervals selected from a range of 1 minute to 20 hours, preferably from a range of 30 minutes to 10 hours, and more preferably from a range of 1 to 4 hours.

In a preferred embodiment of the invention, a current is applied to the at least one pair of bipolar electrodes at a current density selected from the range of 200 to 600 amperes per square meter (preferably from the range of 200 to 400 amperes per square meter).

Those skilled in the art will understand that the apparatus, assemblies, and techniques disclosed below represent those that the inventors have found to work well in the practice of the invention; however, those skilled in the art, in view of the content of this disclosure, will understand that many changes can be made to the specific embodiments disclosed without departing from the scope of the invention, yet still yielding the same or similar results.

Experimental preparation

In all electrode samples used in the following embodiments and comparative examples, a pair of bipolar electrode valve metal substrates were fabricated starting from a 100 mm × 100 mm × 1 mm grade 1 titanium plate. The substrates were degreased with acetone in an ultrasonic bath, then sandblasted and etched with 22% full-boiling hydrogen chloride (HCl).

Catalytic solutions for preparing electrode samples E1, E2a, E2b, and samples C1 to C3 were obtained by dissolving ruthenium and titanium chloride salts in a 10% aqueous solution of hydrogen chloride (HCl). The ruthenium:titanium (Ru:Ti) ratio, expressed as a weight percentage of elements, was 28:72, and the final concentration of ruthenium in each catalytic solution was 45 g/L.

Stir the solution prepared in this way for 30 minutes.

In all electrode samples E1, E2a, E2b, and C1 to C3, the titanium substrate was coated with the above-mentioned catalytic solution using a brush coating method, achieving a ruthenium gain of 0.8 g/m².

After each coat, bake the sample at 500-550°C for 10 minutes.

Repeat the above coating procedure for each sample E1, E2a, E2b, C1 through C3 until the total ruthenium load is achieved according to Table 1 below:

Implementation Example 1

The sample E1 prepared in the experiment was further coated with a topcoat solution obtained by diluting tin (Sn) acetate solution with acetic acid until a final concentration of 40 g/L was achieved. Four coats of the topcoat solution were applied using a brush, resulting in a total tin load of 4.5 g/m². After each coat, the sample was then baked at 500–550 °C for 10 minutes.

After applying the final coat, the sample was post-baked at 500-550°C for 3 hours.

The sample electrode E1 was tested according to the following accelerated testing procedure: At 25°C, a pair of identical electrode samples were placed in a housing with an inlet and an outlet, with a 3 mm gap between the electrodes, and in a 1 liter aqueous solution containing 4 g/L sodium chloride (NaCl) and 70 g/L sodium sulfate ( _{Na₂SO₄ )} .

The electrode pair was operated at a current density of 1000 amperes per square meter, with polarity reversed every minute during the test. The electrode pair was maintained under the test conditions until the battery voltage exceeded 8.5 volts (“accelerated life,” measured in hours for ruthenium per gram per square meter in the catalytic composition).

The results are recorded in Table 2.

E1 lifespan performance (in hours, corresponding to 145 online hours (HOL)) was selected as the target performance for the bipolar electrodes, as listed in Table 2.

At 25°C, with an application rate of 300 amps/square meter and a sodium chloride (NaCl) concentration of 3 g/L in water, the free chlorine (FAC) in this sample can be measured.

Implementation Example 2

Samples E2 (i.e., E2a and E2b) obtained from the experimental preparation were further coated with a topcoat solution. This topcoat solution was obtained by dissolving 80 grams of tantalum chloride ( _TaCl₅ ) in 1 liter of 20% hydrogen chloride (HCl) and stirring the solution at room temperature for 30 minutes. For each E2 sample, one layer of the topcoat solution was applied using a brush, with a total tantalum (Ta) loading of 1 g/m². The samples were first baked at 300-350°C for 10 minutes, and then at 500-550°C for 10 minutes.

Sample E2 was tested according to the same test procedure described in Example 1.

Analysis of sample E2 revealed that the only sample meeting the target performance of E1 was E2b; its performance characteristics are shown in Table 2.

Comparative example 1

Samples C (i.e., C1 to C3) obtained from the experimental preparation were post-baked at a temperature of 500 to 550°C for 3 hours, and then tested according to the test procedure described in Example 1.

Analysis of sample C shows that sample C3 is the only one that meets the E1 target performance; its performance characteristics are listed in Table 2.

The foregoing description should not be construed as limiting the invention, nor should it depart from the scope of the invention. The invention may be used according to different embodiments, and its scope is limited only by the appended patent application.

Throughout this application and the scope of the patent, the word "comprising" and its variations such as "including" and "comprises" are not intended to exclude the presence of other elements, components, or additional procedural steps.

The discussion of patent documents, acts, materials, devices, articles, etc., included in this specification is for the purpose of providing context for the invention only. Prior to the priority date of each of the claims in this application, it is not suggested or implied that any or all of these matters constitute part of the prior art or common knowledge in the relevant field of the invention.

Claims (13)

A chlorination electrolyzer, wherein the catalyst composition, expressed as a weight percentage of elements, comprises 25% to 45% ruthenium and 55% to 75% titanium, comprising: - a housing having an inlet and an outlet suitable for brine; - at least one pair of bipolar electrodes, facing each other and positioned within the housing; characterized in that each of the at least one pair of bipolar electrodes comprises: - a valve metal substrate; - an active coating comprising at least one layer of the catalyst composition disposed on the substrate, the catalyst composition comprising ruthenium and titanium; - a top coating comprising at least one layer of the composition disposed on the active coating, the composition comprising oxides of tantalum, niobium, tin, or combinations thereof. For example, in the chlorination electrolyzer of claim 1, the catalytic composition further includes 2% to 5% doping elements selected from the group consisting of carbide, strontium, ferromium, bismuth, zirconium, aluminum, copper, rhodium, iridium, platinum, palladium, and combinations thereof. For example, in the chlorination electrolytic cell of claim 1 or 2, the active coating has a ruthenium load of 1 to 30 grams per square meter. For example, in the chlorination electrolytic cell of claim 1 or 2, the top coating is composed of tin oxide. For example, in the chlorination electrolytic cell of claim 1 or 2, the thickness of the top coating is 0.5 to 7 micrometers. For example, in the chlorination electrolytic cell of claim 1 or 2, the top coating has a total load of 2 to 6 grams per square meter. For example, in the chlorination electrolytic cell of claim 1 or 2, the valve metal substrate is titanium. A self-cleaning electrochlorination system comprising: - a chlorination electrolyzer as described in any of claims 1 to 7; - an electrolyte comprising an aqueous solution of sodium chloride (NaCl) salt at a concentration of 1 to 30 g/L circulating within the chlorination electrolyzer; and - an electronic system for periodically reversing the polarity of at least one pair of bipolar electrodes and electrically connecting them to the electrodes. A method for manufacturing a chlorination electrolyzer as described in any of claims 1 to 7, comprising the steps of manufacturing each of at least one pair of bipolar electrodes according to the following sequence: a) applying an active coating solution comprising ruthenium and titanium precursors onto a valve metal substrate to obtain a coated substrate; b) baking the coated substrate at a temperature of 450 to 550°C for 2 to 10 minutes; c) repeating steps a) and b) until the desired ruthenium load is achieved; d) applying a top coating solution comprising a precursor comprising tantalum, niobium, tin, or a combination thereof. e) Apply the coating to a coated substrate; f) bake the coated substrate at a temperature of 450 to 550°C for 2 to 10 minutes; f) repeat steps d) and e) until the desired load of tantalum, niobium, tin, or combinations thereof is achieved; and g) perform a final heat treatment at a temperature range of 450 to 550°C; wherein the ruthenium and titanium precursors and the tantalum, niobium, or tin precursors are compounds selected from the group consisting of methoxides, ethoxides, propoxides, butoxides, chlorides, nitrates, iodides, bromides, sulfates, or acetates of metals, and mixtures thereof. A type of chlorination electrolyzer, as described in claims 1 to 7, is used in normal and low salinity tanks for water disinfection via hypochlorite. A method for water disinfection using hypochlorite comprises the following steps: a) circulating an electrolyte comprising an aqueous solution of sodium chloride (NaCl) salt at a concentration of 1 to 30 g/L in at least one chlorination electrolyzer as described in any of claims 1 to 8, the chlorination electrolyzer comprising one or more pairs of bipolar electrodes; b) applying a current to the pairs of bipolar electrodes to generate hypochlorite in the aqueous solution; and c) periodically reversing the polarity of at least one pair of bipolar electrodes during the application of the current. The method described in claim 11, wherein the polarity of the at least one pair of bipolar electrodes is reversed at time intervals selected from one minute to 20 hours. The method described in claim 11 or 12 involves applying a current to the at least one pair of bipolar electrodes at a current density selected from the range of 200 to 600 amperes per square meter.